Thermostatic control for thermionic arc discharge devices



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THERMOSTATIC CONTROL FOR THERMIONIC ARC DISCHARGE DEVICES Filed May 25.1931 Fig. 3

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,4 Inventm:

Albert W. Hull,

MWZZM His Attorney.

P'atentecl Apr. 17, 1934 THERMDSTATIC CONTROL FOR THERMI- ONIC ARCDISCHARGE DEVICES Albert W. Hull, Schenectady, N. Y., assignor toGeneral Electric Company, a corporation of New York Application May 25,1931, Serial No. 539,695

11 Claims.

The present invention relates to electric discharge apparatus, and moreparticularly to devices which carry large currents by means of ionizedgas or vapor.

Many of the devices of this sort contain thermionic cathodes of thefilamentary and indirectly heated type, and are called hot cathodedevices to distinguish them from devices which employ liquid poolcathodes. Mercury vapor ordinarily o is used for the ionizable medium onaccount of its relatively low ionization potential, (10.4 volts).Whenthese devices are employed as relays in contradistinction to simplerectifier-s, an electrostatic control element is added which usuallytakes the form of a perforated grid. Apparatus of this general class isoperated at such ate perature that the pressure of vapor is sufiicientto permit a discharge of arc-like character to form when alternating orinterrupted direct current of suitable voltage is applied to theelectrodes. The are is stopped when the plate voltage is reducedsubstantially to zero. In devices provided with grids, the starting ofthe arc may be controlled by the bias voltage on the grid, but after thearc has started, the grid loses control when ordinary and practical biasvoltages are employed and can neither modulate, limit nor extinguish thearc under these conditions. The starting of the arc in this case may berepeated indefinitely because while the discharge cannot ordinarily beextinguished by the grid, it can be stopped by the removal, periodic orotherwise, of the anode voltage. Upon the re-application or" the latter,the grid voltage again determines whether the arc will start and thus,is able to control the average plate current over a period of time. Allhot cathode devices serve to rectify the alternating current voltagesapplied thereto, but as stated hereinbefore, the devices provided 4 0with grids, in addition, serve to control the av" erage value of therectifier current output.

The operating characteristics or these tubes, such as theircurrent-carrying capacity, also the maximum operating voltage and thelength of time for deionization after the voltage withdrawn, aredetermined to a large extent by the vapor pressure or" the ionizedmedium and this in turn is a function of the ambient temperaturessurrounding the tubes. In order that the op- 50 eration may be uniformover substantial periods of time under conditions of steady appliedvoltages, it is essential that the tubes, or rather certain portions ofthe envelope, as will be explained ';-g.hereinafter, be maintained at aconstant predetermined temperature. These devices, when serving ascontrol or relay units may in practice be positioned in places subjectto large temperature variations, and unless the effects of thesevariations are compensated, the pressure of the vapor within the devicewill change and may lead to arc-back or failure of the device tocommutate properly. It is therefore desirable to employ some form ofautomatic arrangement which will effectively serve to maintain the vaporpressure in these devices substantially constant under all reasonablechanges of ambient andload conditions. An object of the presentinvention is to provide such an arrangement.

It is found that the mercury vapor, hot cathode type of arc dischargedevice lends itself particularly well to the automatic control of itsvapor pressure. The mercury, which is placed in the envelope as aglobule, becomes vaporized and fills the whole tube with mercury vapor,which tends to condense on the coolest portions of the envelope. Theseportions may be and in practice usually are so positioned as to beremote from the region of the discharge, and are shielded from the heatradiated by the discharge so that their temperatures are almost entirelyindependent of the load current. It is evident that these envelopeportions cannot be completely insulated from the heat eiiects of thedischarge, hence of the load current, on account of the heat conductionthrough the material of the envelope, also through the contained vapor.The temperature of these portions, however, largely determines thepressure of mercury vapor throughout the whole envelope and thistemperature is dependent normally upon the temperature or the air and tosome extent upon the load current. Accordingly, I propose to apply theautomatic temperature regulation to the coolest portion of the envelopeof such devices. One of the many forms which my invention may take isthat in which an arrangement is provided automatically to add heat tothat part of the envelope in which the liquid mercury and the condensedvapor collects. The operation of my improved arrangement is such thatthe addition of heat is controlled by the temperature of this part hithe envelope in such mamier as to maintain the temperature of this partconstant. The invention will be more clearly understood when referenceis made to the drawing in which Fig. 1 represents a cross section,partly broken away, of a tube improved in accordance with my invention,and Figs. 2 and 3 show a modified construction.

Referring to Fig. 1, numeral 1 designates a glass envelope whichterminates in a stem 2 and press 3. The latter supports an indirectlyheated cathode 4 which contains an axially positioned filament 5connected to the upper end of the cathode. Conductors 6 and 7 areconnected to the lower ends of the cathode and filament respectively,said conductors terminating in contact pins 8 which are secured to thebase 9. The stem 2 provides a support for an electrostatic controlmember of the usual form, of which only a portion 10 of each supportingupright is indicated. These uprights may be secured to the stem by meansof a clamp 11; It will be understood that in addition to the cathode andcontrol members referred to, there is also an anode (not shown). A lead12 for the control member is connected to a contact pin (not shown) inthe base. A connection is taken from the anode, preferably through theupper portion of the envelope. The envelope contains a quantity ofmercury 13 which settles in an annular reservoir at the lower end of thetube. When voltages of suitable character and magnitude are applied tothe electrodes, a discharge of arc-like character is produced betweenthe electrodes, as a result of the ionization of mercury vapor, the arebeing characterized by a nearly flat voltampere characteristic. Asstated hereinbefore, the initiation of the discharge may be controlledby utilizing for the anode energy supply, a source of alternatingcurrent or interrupted direct current, and in addition, if desired, bycontrolling the bias voltage on the grid, as is well understood in theart. Devices of this character have been described by me in an articleentitled Hot cathode thyratrons in the General Electric Review, vol. 32,No. 2, April 1929, pages 213 to 223 inclusive.

In apparatus of this kind, particularly in devices employing vapor forthe ionizable medium, the matter of envelope temperature becomes afactor of great importance. The vapor is continually going through aprocess of condensation and re-evaporation during which the temperatureof the coolest portion of the envelope determines the vapor pressurethroughout the envelope. When the ambient temperature changes as in thecase, for example, of a device presented alternately to sunlight andshadow, or summer and winter conditions, also when the load changes, theresulting changes in vapor pressure may cause marked variations in theoperation of the tube.

In accordance with my invention, I propose to maintain the operation ofsuch a device substantially constant by controlling the temperature ofthe coolest portion of the envelope and for this purpose, a heater 14 isemployed near the lower end of the tube, the heater consisting ofresistance wire which is attached at one end to the cathode lead 6 andat the other end, to a thermostatic strip 15 of bimetallic material. Theheater and strip members conveniently are contained in the base 9 or ina socket (not shown) into which the base is inserted, in order toconserve heat and to protect the elements from injury. As exemplified,the base is hollow and has secured thereto on the interior, aninsulating strip 16 which carries a contact member 17 electricallyconnected by lead 18 to the filament lead '7. The thermostatic strip maybe secured in any suitable manner to a body of insulating material 19which closes the lower end of the base. The contact member is positionednear the upper'end of the thermostatic strip so that when the latterbends or flexes under the influence of heat, connection is made with thecontact member, thus connecting the heater directly across the leads 6,7. Electrical energy may be supplied to the heater conveniently from thesame source as energizes the filament. It is evident that the heater, byreason of its position, serves to elevate the temperature of the loweror cooler portion of the envelope which contains the condensate 13. Thethermostatic strip 15 is located near the periphery of the base 9, andis so far removed from the arc discharge as to respond almostexclusively to changes of temperature which occur about the lower end ofthe tube. When the heater becomes energized by reason of a lowering ofthe ambient temperature or a reduction in load, additional vapor isproduced from the mercury condensate, which serves to increase andrestore to the desired value, the pressure of the ionizable mediumwithin the active region of the discharge. The increase of vaporpressure restores the electrical characteristics of the tube to theiroriginal value, notwithstanding the lower ambient temperature andreduced load, thus maintaining the operation of the device substantiallyuniform. When the vapor pressure reaches the predetermined value as theresult of the heat supplied by member 14, the thermostatic strip bendsin the direction opposite to that previously described and opens theheater circuit, thereby allowing the lower end of the envelope to cool.

An improvement on the broad invention claimed herein is disclosed andspecifically claimed in the patent application of John H. Payne, SerialNo. 541,083 filed May 29, 1931, entitled Thermostatic control forelectron discharge devices, and assigned to the same assignee as thepresent application. This improvement is shown in Figs. 2 and 3 and noclaims specific thereto are made in the present application. The device20, which may be a rectifier, with or without electrostatic control, andcontaining mercury vapor or other ionizable medium, is placed in anenclosure 21 provided with ventilating doors or shutters 22. The latterconveniently are arranged to swing on horizontal axes 23 and aresuspended in such a manner as normally to move inwardly in the absenceof restraint. However, the doors are maintained in the closed positionby means of the U-shaped thermostatic strip 24 which is secured to theenclosure in any suitable manner and at a position near the coolestportion of the device. As shown, a projection 25 is provided on eachdoor which rests against the strip so that when the thermostatic loopopens due to increase of temperature, the, doors fall of their ownweight, leaving openings in the side of the enclosure. The normalcathode heat radiation including the effects of load current, maintainsthe enclosure with the doors closed, at a sufiiciently high temperaturein the coldest operating ambient condition. If this radiation is notsuificient, a heater (not shown) may be provided. As soon as thetemperature in the enclosure rises above the desired value, the doorsautomatically and gradually open, finally assuming a stationary positionsuch as to allow a sufficient quantity of air to enter and cool thelower portion of the tube, so that the desired temperature ismaintained. This arrangement differs from that previously described inthat the desired temperature regulation is obtained without thenecessity of supplying a controlled amount of additional heat to thetube.-

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is,

1. A thermionic discharge device including an envelope, and containing aplurality of electrodes and a material adapted to be vaporized, aportion or" said enveloped beingadapted to operate at a temperaturesufficie tly high to maintain the vaporizable material in a vaporouscondition and another portion of the envelope being adapted to operateat a temperature sufficiently low to cause condensation of a portion ofthe vapor, means including a temperature responsive device forautomatically maintaining the temperature of the cooler portion only ofthe envelope at a predetermined value whereby the pressure of the vaporis maintained constant regardless of ambient temperature variations.

2. An electric discharge device comprising an envelope containing aplurality of electrodes and a vapor adapted to be ionized in order tosupport an arc-like discharge, a portion oi said envelope being adaptedto operate at a temperature suiliciently high to maintain the ionizablemedium in a vaporous condition and another portion of the envelope beingadapted to operate at a temperature sumciently low to cause condensationof a portion of the vapor, and means including a temperature responsivedevice for automatica regulating the rate at which the vapor iscondensed and re-evaporated during operation in response to variationsof envelope temperature, whereby the pressure or" the vapor ismaintained substantially constant notwithstanding the va riations ofenvelope temperature 3. An electric discharge device comprising anenvelope and containing a plurality of electrodes and a vapor adapted tobe ionizec order to support an arc-like discharge, a portion of saidenvelope being adapted to operate at a temperature sufficiently high tomaintain the ionizable medium in a vaporous condition and anotherportion of the envelope being adapted to operate at a temperaturesufiiciently low to cause condensation of a portion the vapor, meanspositioned at the cooler portion of the envelope and responsive totemperature variations of the envelope for automatically regulating thepressure of the vapor in response to variations of envelope temperature,said means oompr' heater in heat-transferring relation to the vaporcondensate and which is energized when the velope temperature be omeslower than a predetermined temperature.

4. An electric discharge device comprising envelope containing athermionic cathode, an anode and a vapor adapted to be ionized in orderto support an arc-like discharge, a portion of said envelope beingadapted to operate at a temperature suficiently high to maintain theionizable medium in a vaporous condition and another portion of theenvelope operating at a temperature sufiiciently low to causecondensation of a portion or" the vapor, and means positioned at thecooler portion of the envelope for regulating the pressure of the vaporin response to the variations of envelope temperature, said meanscomprising a heater, also a thermostatic member for automaticallyconnecting the heater to a source of electrical energy when the envelopetemperature becomes lower than a predetermined temperature.

5. An electric discharge device of the hot cathode and vapor type andincluding an enevolpe containing a plurality of electrodes andvaporproducing material, a base for said device, and means responsive totemperature variations and secured within the base, for regulating theter perature of the vapor-producing material in response to thevariations of envelope temperature whereby the pressure of the vapor insaid envelope is maintained constant.

6." An electric discharge device including an envelope which contains aplurality of electrodes and a quantity of mercury, said mercury beingadapted to vaporize during operation and to support an arc-likedischarge, a portion of said envelope being adapted to operate at atemperature sufficiently high to maintain the mercury in a vaporouscondition and another portion of the envelope being adapted to operateat a temperature sufficiently low to cause condensation of a portion ofthe vapor, and means responsive to temperature variations and positionednear the cooler portion only of the envelope to control the rate atwhich the mercury condenses on and reevaporates from the said portion ofthe envelope, in respnse to envelope temperature variations.

7. An electric discharge device including an envelope which contains anindirectly heated cathode, a cathode heater, an anode and a quancity ofmercury, said mercury being adapted to vaporize during operation and tosupport an arc- 'l :e discharge, a portion of said envelope beingadapted to operate at a temperature sufiiciently high to maintain themercury in a vaporous condition and another portion of the envelopeoperating at a temperature sufficiently low to cause condensation of aportion of the vapor, and means responsive to temperature variations andpositioned near the cooler portion only of the envelope to controlautomatically the rate at which the mercury condenses on the saidportion of the envelope in response to the temperature variations, saidmeans comprising a heater connected eiiectively during operation betweenthe cathode and the cathode heater.

8. A gaseous conduction device of the hot cathode iype and including anenvelope containing a plurality of electrodes and vapor producingmaterial, a support m mber for the device in thermal contact therewith,said member containing a heater in heat-transferring relation to saidmaterial, and temperature responsive means for regulating thetemperature of the heater in response to va tions of envelopetemperature.

9. A gaseous conduction device of the hot cathode type and including anenvelope containing a plurality of electrodes and vapor-producingmaterial, a for said envelope, said base being hollow and containing aheater in heat-transferring relation to said material, also containingmeans responsive to temperature variations for energizing the heaterwhen the ambient temperature about the envelope becomes lower than apredetermined temperature.

19. A gaseous conduction device of the hot cathode type and including anenvelope containing a plurality of electrodes and vapor-producmaterial,a base for said envelope, said base being hollow and containing a heaterin heattransferring relation to said material, also containing a relayin heat responsive relation to said envelope, relay being adapted tocause the energizaticn of said heater when the envelope becomes lowerthan a predetermined temperature whereby the pressure of the vapor ismaintained substantially constant.

11. A gaseous conduction device including an envelope which contains anindirectly heated cathode, a cathode heater, an anode and vaporproducingmaterial, a base for said envelope, said base bein hollow and containinga heater in heat-transferring relation to said material,

leading-in conductors when flexed in response to a change of envelopetemperature whereby the energization of the heater is controlled inaccordance with envelope temperature and the vapor pressure in theenvelope is maintained substantially constant.

ALBERT W. HULL.

